Id proteins are dominant negative regulators of basic helix-loop-helix transcription factors. Previous work in our laboratory has shown that constitutive expression of Id-1 in SCp2 mouse mammary epithelial cells inhibits their differentiation and induces proliferation, invasion, and migration. Id-1 expression also correlates with the invasive and aggressive potential of human breast cancer cells. However, little is known about Id-1 target genes that are important for regulating normal and transformed breast epithelial cell phenotypes. Now we report the cloning of a novel zinc finger protein, Zfp289, using degenerate primers to specifically amplify cDNAs from Id-1-transfected SCp2 cells. Zfp289 has homology with a yeast zinc finger protein, the GTPase-activating protein Gcs-1, which was initially identified as a gene required for the re-entry of cells into the cell cycle after stationary phase growth. Zfp289 mRNA expression pattern correlates with Id-1 expression in SCp2 mammary epithelial cells under various experimental conditions as well as in the mouse mammary gland at different stages of development. It is predominantly present in the cytoplasm of the cells as evident from green fluorescent protein fusion protein localization. SCp2 mammary epithelial cells with constitutive expression of Zfp289 have a higher S-phase index, compared with control cells, when cultured in a serum-free medium. We conclude that the novel zinc finger protein Zfp289, which may represent the mammalian homologue of Gcs-1, is potentially an important mediator of the Id-1-induced proliferation pathway in mammary epithelial cells.